Great guide. Thanks a lot for it. I read that It might be possible to glitch or farm for red gems so I did a little of those and figured missing a few from the guide wouldn't matter. I got near the end and needed more than were left (post-5-2/Christopher), I chose to restart the chapter and using this guide the last three gems were my last three upgrades. Got really lucky there. I'd recommend people get just about every one they can normally and buy others as a safety measure. If I'd have needed one more gem I would have had to collect them in another playthrough.

Yeah, seriously. I completed the game on easy last night, my first playthrough. Ended on 79 red gems. For my last visit to Christopher, I had 24 white gems left over... so I was only 21 white gems away from the achievement. So, basically, every red gem counts. Not sure what I will do now... not too confident about trying to get 80 red gems on normal.